Compact seismic source for low frequency, humming seismic acquisition

1. A compact seismic source for seismic acquisition, generating a humming, low frequency seismic signal at any one time, the source comprising:

a casing defining a fluid tight chamber and comprised of a first casing section and a second casing section, wherein the first casing section has a first mass and the second casing section has a second mass, and wherein the first mass is within approximately +/− 10% of the second mass;

a low-frequency reciprocating drive disposed within the fluid tight chamber that, in operation, reinforces the natural reciprocating oscillation of the first and second casing sections relative to one another at a low seismic frequency, wherein the low-frequency reciprocating drive comprises a push-pull rod positioned within the casing and affixed to the first casing section;

a sliding seal engaged between an outer surface of the first casing section and an inner surface of the second casing section, wherein the sliding seal, the outer surface of the first casing section, and the inner surface of the second casing section are disposed about the push-pull rod;

wherein the size of the compact seismic source is substantially smaller than the wavelength of the low seismic frequency;

a gas reservoir operationally coupled to the chamber; and

a controller electrically connected to the gas reservoir and the excitation actuator to control their operation to control a resonating frequency for the compact seismic source.

2. The compact seismic source of claim 1 , wherein the first and second casing sections each include a hemispherically-domed end.

3. The compact seismic source of claim 1 , wherein the reciprocating drive comprises:

an excitation actuator disposed in the second casing section;

wherein the push-pull rod is mechanically linked to the excitation actuator at one end thereof and is affixed to the first casing section at a second end thereof.

4. The compact seismic source of claim 3 , further comprising:

at least one bearing for the push-pull rod disposed in the first casing section; and

a guide for the push-pull rod disposed in the second casing section.

5. The compact seismic source of claim 1 , further comprising a plurality of fluid-tight chambers within at least one of the first casing section or second casing section, the plurality of fluid-tight chambers in fluid communication with each other via a controllable valve.

6. The compact seismic source of claim 5 , wherein volumes of the fluid-tight chambers are arranged in a powers-of-two distribution.

7. The compact seismic source of claim 1 , wherein the source radiates a humming, low-frequency seismic signal omni-directionally.

8. A method for use in marine seismic surveying, comprising:

deploying a low-frequency seismic source, compact on the scale of the wavelength of its radiated signals, the low-frequency seismic source comprising:

a casing defining a fluid tight chamber and comprised of a first casing section and a second casing section of roughly equal mass, wherein the first and second casing sections each include a hemispherically-domed end, wherein the first casing section has a first mass and the second casing section has a second mass, and wherein the first mass is within approximately +/− 10% of the second mass;

a low-frequency reciprocating drive disposed within the fluid tight chamber that, in operation, reinforces the natural reciprocating oscillation of the first and second casing sections relative to one another to generate the low-frequency, humming seismic signal, wherein the low-frequency reciprocating drive comprises a push-pull rod positioned within the casing and affixed to the first casing section; and

a sliding seal engaged between an outer surface of the first casing section and an inner surface of the second casing section, wherein the sliding seal, the outer surface of the first casing section, and the inner surface of the second casing section are disposed about the push-pull rod;

generating a humming, low-frequency seismic signal from the low-frequency seismic source by oscillating the push-pull rod and the first casing section relative to the second casing section and sliding the sliding seal along the outer surface of the first casing section or the inner surface of the second casing section;

omni-directionally radiating the humming, low-frequency seismic signal;

deploying a receiver array; and

acquiring seismic data from reflections of the low-frequency seismic signal.

9. The method of claim 8 , wherein omni-directionally radiating the low frequency seismic signal includes omni-directionally radiating a low-frequency seismic signal in the range of 0.5-2 Hz.

10. The method of claim 8 , wherein deploying a low-frequency seismic source further comprises:

deploying a gas reservoir operationally coupled to the fluid tight chamber; and

deploying a controller electrically connected to the gas reservoir and the excitation actuator to control their operation.

11. The method of claim 8 , wherein deploying the low-frequency seismic source includes deploying a low frequency, seismic source at a depth of 25 meters or greater.

12. The compact seismic source of claim 1 , further comprising a bulkhead positioned within the fluid tight chamber, wherein the bulkhead includes a port that is configured to be selectively opened to allow fluid communication through the bulkhead.

13. The method of claim 8 , further comprising:

opening a port of a bulkhead positioned within the fluid tight chamber;

flowing fluid through the bulkhead as a result of opening the port; and

changing a resonant frequency of the seismic source as a result of flowing fluid through the bulkhead.